For many years sandwich structures have been produced comprising honeycomb or foam cores both surfaces of which are covered by a panel composed of several layers of fabric impregnated with polymerized resin.
These sandwich structures are extremely light while having good mechanical resistance to compression and flexion. However, the fact that they are constructed by combining three separate components limits their cohesion and it is not possible to adapt the mechanical properties of the structure to precise specifications. Furthermore, traditional sandwich structures have to be planar.
There have also been proposals for a textile structure comprising a core covered by two layers produced in a single operation. More precisely, proposals have been made to produce a composite sandwich structure composed of a velvet construction consisting of two foundation warps and one pile warp before the latter is cut to obtain two separate pieces of fabric.
Although this type of textile structure has the advantage of being easily produced in a single operation and of giving greater cohesion than a standard sandwich structure, the construction of non-planar panels remains virtually impossible in practice due to the use of a weaving method that requires the warp and weft threads to be kept taut. This phenomenon is exaggerated by the simultaneous construction of two textile surfaces and a network of threads connecting the said surfaces.
Moreover, in a textile structure produced using this method it is very difficult to control the orientation of the linking threads constituting the core of the structure and thus to adapt the mechanical properties of the structure to precise specifications.
Document number WO-A-92 13125 also proposes to construct thermal insulation panels using a textile armature constructed in a single knitting operation and comprising two knitted layers linked to one another by a network of linking threads.
Compared with a woven textile structure, this type of knitted textile structure has the advantage of being capable of being shaped, making it possible to produce non-planar panels with single or multiple curvature. The knitting technique causes formation of stitches that give a certain degree of flexibility to the layers of the textile structure.
However, the structure described in document number WO-A-92 13125 is produced using a cycle that consists in knitting one course of stitches on each layer and linking them together with a course of linking threads on a double-bed machine. This results in the linking threads being oriented virtually at a right angle to the two textile surfaces forming the layers of the structure in the direction of its warp. The textile structure described in this document is therefore incapable of adapting its mechanical properties to precise specifications by giving total control over the orientation of the linking threads.
In US document No. U.S. Pat. No. 5,385,036 a textile structure is knitted using a Raschel-type knitting machine with two needle-bars. All the needles are constructed as part of the bar and make the same movements simultaneously.
The textile structure produced by this type of machine comprises two layers connected by linking threads. However, one of the networks of linking threads has to be perpendicular to the planes of the layers. Moreover, in the structure described most of the linking threads are attached in alternating course to one of the layers.